FIG. 1 shows the basic design of a gas turbine blade with trailing edge cooling means, which consists in radial flow channels connected via flow bends to form a multi-pass serpentine.
The blade 10 of FIG. 1 comprises an aerofoil 11 extending in radial direction with a leading edge 15 and a trailing edge 16, a suction side 17 and a pressure side 18. At the lower end of the aerofoil 11 a platform 12 is provided for defining the inner wall of a hot gas path. Below the platform 12, the blade 10 has a shaft 13, which transforms into a root 14 with a well-known fir tree profile. In the interior of the aerofoil 11 a plurality of radial, parallel flow channels are provided, which guide a cooling flow 20 in a serpentine way. A trailing edge ejection region 21 is designed to establish a pressure side bleed 28 for cooling the trailing edge 16.
This arrangement can lead, under certain operating conditions, to excessively high coolant and metal temperatures at the blade trailing edge 16. Particular attention should, therefore, be paid to the cooling of the trailing edge region. For efficiency purposes, the trailing edge 16 should remain as thin as possible and the trailing-edge cooling should be restricted only to what is required. Also the cooling of the trailing edge 16 should be uniform to avoid mechanical integrity penalties. The cooling design of the trailing edge region becomes more critical when using re-cooled cooling air (reduced cooling air requirements).
Document EP 1 707 741 A2 discloses a hollow cooled airfoil, which includes an array of radially-spaced apart, longitudinally-extending partitions defining a plurality of cooling channels there between. A plurality of aft pins is disposed in at least one of the cooling channels. Elongated turbulators are disposed in at least one of the cooling channels, and are oriented at an angle to a longitudinal axis of the vane such that an aft end of each of the turbulators is closer to an adjacent partition than a forward end of the turbulator. The vane includes an array of radially-spaced apart, longitudinally-extending lands and longitudinally-extending dividers which define plurality of trailing edge slots there between. Each of the trailing edge slots has an inlet in fluid communication with a trailing edge cavity and an axially-downstream exit in fluid communication with the vane's trailing edge. The dividers have an axial length less than an axial length of the lands. The mixture of aft pins, turbulators, dividers, partitions and lands is quite complicated, such that an optimization of the cooling flow is extremely difficult.
Document U.S. Pat. No. 5,288,207 A teaches a turbine airfoil having a baffleless cooling passage for directing cooling fluid toward a trailing edge. Various construction details are developed which provide axially oriented, interrupted channels for turning a flow of cooling fluid from a radial direction to an axial direction. In a particular embodiment, the turbine airfoil has a cooling passage including a plurality of radially spaced walls, a plurality of radially spaced dividers downstream of the walls, and a plurality of radially spaced pedestals positioned axially between the walls and dividers. The walls and dividers define channels having an axial interruption permitting cross flow between adjacent channels. The cross flow minimizes the adverse affects of a blockage within a sub-channel between adjacent walls. The pedestals are aligned with the sub-channels such that cooling fluid exiting a sub-channel impinges upon the pedestal. The combination of interrupted channels and a pedestal or pin within each channel is on one hand simple but on the other hand not flexible enough to optimize the cooling requirements for different areas of the trailing edge region.
Document EP 1 340 884 A2 discloses an airfoil for a gas turbine engine nozzle, said airfoil comprising a first sidewall and a second sidewall connected at a trailing edge such that a cavity is defined there between, each said sidewall extending radially between an airfoil root and tip, said first sidewall comprising a plurality of slots extending towards said trailing edge, said airfoil further comprising a plurality of pins and at least a row of turbulators, said pins extending between said first and second sidewalls, said turbulators extending between said pins and said slots. The combination of slots, pins and turbulators is of low flexibility, as all slots, pins and turbulators have essentially the same geometry and dimensions.
Document EP 1 715 139 A2 teaches an airfoil having a pressure side wall having a span-wise extending downstream edge and a suction side wall having a downstream trailing edge, said downstream edge being spaced from said trailing edge to expose a back surface of said suction side wall, comprising; a span-wise cooling air cavity defined between said pressure and suction side walls; a trailing edge region disposed downstream of said cavity; a span-wise extending slot fluidly interconnecting said cooling air cavity to said trailing edge region; wherein said slot includes a plurality of pedestals extending between said suction side and pressure side walls and through said slot, said pedestals being disposed in span wise extending rows with the most upstream row having pedestals of greater cross sectional dimension and those more downstream rows having pedestals of lesser cross sectional dimension. This disclosed distribution of cross sectional dimensions along the cooling flow direction is intended to provide for a smooth transition and pressure drop, resulting in a more continuous heat transfer coefficient. However, the cooling of the fragile trailing edge is far from being optimal.
Document WO 2010/086419 A1 discloses a cooled vane for a gas turbine comprising an airfoil which extends between a leading edge and a trailing edge in the direction of flow and is respectively delimited by a wall on the suction side and the pressure side. Said walls enclose an interior, inside which cooling air flows to the trailing edge in the direction of flow and is discharged in the region of the trailing edge. In order to reduce aerodynamic losses on the trailing edge and the amount of cooling air used in such a vane, the wall on the pressure side ends at a distance from the trailing edge in the direction of flow so as to form a lip on the pressure side such that the cooling air is discharged from the interior on the pressure side. Furthermore, at a distance from the trailing edge, the interior is subdivided into a multitude of parallel cooling ducts causing a great drop in pressure by means of a multitude of ribs that are oriented parallel to the direction of flow, turbulators are arranged inside the cooling ducts to increase the cooling effect, and a plurality of flow barriers is distributed transversely to the direction of flow within the flow path of the cooling air at a short distance upstream of the point where the cooling air is discharged from the interior. A plurality of pins with identical dimensions is distributed between the cooling ducts and the flow barriers.
Document EP 1 548 230 A2 discloses a cooled blade for gas turbine, comprising a radially extending aerofoil with a leading edge, a trailing edge, a suction and a pressure side, a plurality of internal flow channels connected via flow bends to form a serpentine for a coolant flow, a trailing edge ejection region being connected over its entire length with a bleed opening, whereby the cooling flow from the trailing edge passage to the bleed opening is mainly determined by a staggered field of pedestals, which is provided between said bleed opening and said trailing edge passage. The lateral dimension of the pedestals increases in the coolant flow direction.